We look for ourselves in the stars. Perhaps the major discovery of the last decade is that our solar system is not the only system of planets around stars. To date, more than 235 planets have been discovered orbiting other suns. We call them extrasolar planets, or exoplanets, using the Latin sense of “extra” as “outside” or “beyond”.

Gliese 581
Digitized Sky Survey image of the star Gliese 581, a dim red dwarf 20.4 light-years away toward the constellation Libra. ESO/DSS

Discovering exoplanets is not an easy task. The planets of our solar system are visible because of the sunlight that they reflect. Unfortunately, nearly all planets that orbit other stars are far too faint relative to their stars for current astronomical telescopes to observe them directly as faint points of light next to their much brighter stars. Instead, astronomers use a variety of techniques to indirectly infer the presence of these extrasolar planets.

The method by which most of the known exoplanets have been discovered is the radial-velocity (or Doppler) technique. The mass of the orbiting planet causes the central star to be pulled around in an orbit. Astronomers detect the resulting small, periodic shifts in the apparent motion of the star. By measuring the shape of the resulting Doppler curve over time, they are able to deduce a lower limit on the mass of the planet and estimate the separation of the planet from the star.

Most of the extrasolar planets discovered using this method are Jupiter-like, or much more massive than the gas giant. They also orbit perilously close to their host stars, in locations that would likely prevent the development of life. However, on April 24, 2007, scientists using the 142-inch telescope of the European Southern Observatory in Chile announced the discovery of the most Earth-like extrasolar planet found to date.

The new world is called Gliese 581 C, after its star, Gliese 581, a red dwarf located 20.4 light-years away that is about one-third as massive as the Sun. Gliese 581 C is believed to be a rocky planet, about 50 percent bigger than Earth and about five times more massive. The “super-Earth” orbits 15 times closer to its star than Earth is to the Sun, but because red dwarfs are about 50 times dimmer than the Sun and much cooler, this means that Gliese 581 C is well within the star’s habitable zone! The habitable zone is the space around a star where liquid water and life can exist on a planet’s surface.

Astronomers could not observe Gliese 581 C directly, because it is invisible in the glare of its sun, but the planet’s star, Gliese 581, is readily visible even through a small telescope! The red dwarf shines at magnitude 10.6 and can be found about 2° northeast of Beta Librae, the brightest star in the constellation Libra. Even though Gliese 581 is rather faint, it is also very red, and it is possible that color alone will identify the star for you once you have located the field.

Finder map – field width 7°, stars to magnitude +11.